Abstract
This review summarizes the interactions between three major bacterial groups, Rickettsia sp., Bartonella sp. and Yersinia pestis, the flea vectors and the diverse gut microbiota of fleas and highlights open questions. The focus is on the plague pathogen, Y. pestis, which adapted to transmission by fleas several thousand years ago. This caused one of the deadliest infectious diseases known to mankind, and the three pandemics resulted in an estimated 200 million deaths. In the vector, Y. pestis resists the adverse conditions, like other numerous bacterial species. Rickettsia sp. and Bartonella sp. as well as Y. pestis induce specific changes in the microbiota. The presence of bacteria in the ingested blood activates the production of antimicrobial proteins and reactive oxygen species, which normally have no effect on the development of Y. pestis. This bacterium infects mammals by different modes, first by an early-phase transmission and then by biofilm-mediated blockage of the foregut. Both interfere with blood ingestion and lead to reflux or regurgitation of intestinal contents containing Y. pestis into the bite site. Blockage of the gut leads to more attempts to ingest blood, increasing the risk of transmission. The lifespan of the fleas is also reduced. As Y. pestis is still endemic in wildlife in many regions of the world and human infections continue to occur in limited areas, studies of the interactions are needed to find new ways to control the disease.